Studies are in progress evaluating the nature of interactions between heparin-antithrombin and PF-4. These studies will help explain the mechanism by which PF-4 exerts its anti-heparin effect. A radio-immunoassay for PF-4 has been developed and will be used to study patients with thromboembolic disorders. Detailed studies of patients with acute myocardial infarction, transient coronary ischemia and patients with prosthetic cardiac valves are planned. Results to date demonstrate an increased plasma PF-4 level in patients with acute infarcts and a majority of patients with prosthetic and porcine valves.